The disclosure herein relates to subject matters contained in Japanese Patent Application No. 9-141948 filed on May 30, 1997, based on which a benefit of the priority of the Paris Convention is claimed in the present application, and thus the entirety of the Japanese Patent Application is expressly incorporated herein.
The present invention relates to a process of producing a thin ring shaped resin bonded magnetic member based on for example Fexe2x80x94Bxe2x80x94R (wherein R is Nd and/or Pr) magnetic powder in which member the magnetic powder is densely filled, and also to such a magnetic member which is produced by the process. The member has a wall thickness of for example not greater than 1 mm. The magnetic member may be used in a disc driving motor installed in a personal computer which motor is required to be compact and to produce a larger power and/or to consume less electric power.
The present process uses a molding apparatus which is applicable to continuous (or successive) compression molding of a resin bonded magnetic member and which mechanically or hydraulically applies a pressure, and the process comprises the following basic steps of mold filling, compressing and mold releasing.
Magnet has been developed in a resin bonded magnet and a sintered magnet separately under the consideration of their performances and production costs with respect to their forms when they are used. In general, in order to ensure the priority of an Fexe2x80x94Bxe2x80x94R based resin bonded magnet in its practical application, it is advantageous that such magnet is intended to be formed into a thin ring shape which is difficult for the sintered magnet to be formed into. Magnetic characteristics of an Fexe2x80x94Bxe2x80x94R based resin bonded magnetic member depend on an overall density of the member and also the magnet itself which constitutes the member. Thus, it is required to increase the density as much as possible with considering the production cost of the member.
In the molding production of the thin (for example a thickness of less than 1 mm) ring shaped resin bonded magnetic member based on Fexe2x80x94Bxe2x80x94R magnetic powder as described above, it is generally difficult to uniformly fill the magnetic powder in a mold cavity, to densely compress the powder, or to ensure good dimensional accuracy of a green compact. Further, it is also difficult to, in a readily usable form as a magnetic member, ensure a precise size of the member and to make the member present characteristics of the magnetic powder itself.
Hitherto, magnetic powder which contains an epoxy resin composition (about 1-6% by weight) has been used for the production of a ring bonded magnetic member based on Fexe2x80x94Bxe2x80x94R or Smxe2x80x94Co.
For example, anisotropic magnetic powder is used together with an epoxy resin composition which is in a liquid state at a room temperature for the production of the resin bonded Fexe2x80x94Bxe2x80x94R or Smxe2x80x94Co based magnetic member. However, such an epoxy resin composition adversely affects on fluidity (or free-flowability) of the magnetic powder, so that it is difficult to uniformly, fill a mold cavity with its fluidity. Further, it is not easy to handle a green compact during mold releasing and curing of the resin composition thereafter because of less mechanical strength of the green compact. Thus, it is difficult to ensure and keep good dimensional accuracy of the magnetic member as a final product.
In order to overcome the above problem, Japanese Patent Kokai Publication No. 60-194,509 discloses a process in which magnetic powder is coated with an epoxy resin composition which is in a solid state at a room temperature, and provided with suitable powder fluidity; the magnetic powder is filled in a mold cavity at a temperature below a softening point of the epoxy resin composition; the magnetic powder is compressed while a mold is heated to a temperature above the softening point; and the mold is cooled followed by releasing a molded magnetic member.
The above process allows the magnetic powder to be compressed as densely into the magnetic member as in the case wherein the epoxy resin composition which is in a liquid state at a room temperature. In addition, when a temperature of the whole mold is below the softening temperature of the epoxy resin composition during the filling step, uniform filling of the magnetic powder into the mold cavity is possible so that a green compact produced by compression while heating has improved handling properties. Thus, it is possible to ensure densely filling of the magnetic powder and also to keep good dimensional accuracy required for the magnetic member which is used for actual applications.
However, such a process requires to heat and then cool the whole mold relative to the softening point every molding cycle, and the mold has a heat capacity which is much larger than that of the bonded magnetic member. Such a large heat capacity increases periods required for heating and cooling. Thus, a cycle period for the molding (i.e. a total time for filling, compressing while heating, cooling and releasing steps) is considerably increased. Therefore, the process disclosed in Japanese Patent Kokai Publication No. 60-194,509 is not suitable for commercial production of the ring shaped resin bonded magnetic member in a large scale.
Japanese Patent Kokai Publication No. 63-194,312 discloses a process of producing an Fexe2x80x94Bxe2x80x94R based resin bonded magnetic member in which powder composite of magnetic flake-like powder (of which more than 50% by weight has a particle size above 75 xcexcm) which is coated with an epoxy resin composition is filled into a mold cavity by means of its fluidity, the powder composite is compressed to obtain a green compact, the green compact is released from the cavity, and then the green compact is heated to cure.
The process allows a stably continuous operation and produces the magnetic member having an improved dimensional accuracy. However, in the process, the solid epoxy resin composition does not start sufficient plastic flow so as to increase a filling density of the magnetic powder even when the composite is compressed. Further, since many particles of the magnetic powder cohere in the mold, it is impossible to produce an anisotropic magnetic member with highly orientated.
It is an object of the present invention to provide a process of producing a ring shaped resin bonded magnet (or a ring shaped resin bonded magnetic member) in an industrial large scale which member having a good dimensional accuracy, a large density and a proper mechanical strength.
The other object will be clear from the following description.
There is provided a process of producing a ring shaped resin bonded magnet or magnetic member comprising the steps of:
(1) mixing magnetic powder, in particular Fexe2x80x94Bxe2x80x94R (wherein R is Nd and/or Pr) based magnetic powder, with a solution of an organic solvent which contains at least one heat curable resin composition which is in a solid state at a room (or a normal) temperature and which is capable of coating the powder so as to form a mixture of the magnetic powder and the solution;
(2) removing, in particular removing by evaporating, the organic solvent from the mixture formed in the step (1) (optionally followed by dissociating composite blocks made of cohering magnetic power with the resin composition which are formed by the removal of the organic solvent) so as to obtain the magnetic powder which is composite with the resin composition, and then classifying the magnetic powder so as to obtain a magnetic powder composite having a predetermined particle size range made of the magnetic powder and the resin composition (preferably which composite is substantially coated with a film of the resin composition);
(3) mixing the magnetic powder composite with at least one lubricant so as to obtain a feed mixture for molding made of the magnetic powder composite and the lubricant;
(4) filling an annular cavity of a mold with the feed mixture for molding by means of its powder fluidity;
(5) heating an inner peripheral surface of the annular cavity so as to transfer heat to the feed mixture through the inner peripheral surface, whereby heating the feed mixture to a temperature not lower than a softening temperature and below a curing temperature of the resin composition, and compressing the filled feed mixture after or during the heating;
(6) cooling the inner peripheral surface of the annular cavity so as to transfer heat from the heated and compressed feed mixture through the inner peripheral surface, whereby cooling the feed mixture to a temperature below the softening temperature of the resin composition (preferably to a room temperature) so as to obtain a green compact having a ring shape;
(7) releasing and taking out the green compact from the cavity, and
(8) heating the green compact to a temperature not lower than the curing temperature of the resin composition.
By the above process, the ring shaped resin bonded magnetic member is produced in an industrial large scale which member has a high density, a sufficient mechanical strength and good dimensional accuracy even though the member has a thin wall (which means that a wall constituting the ring shape is thin, for example, a thickness of about 1 mm). It is noted that the term xe2x80x9cringxe2x80x9d includes in addition to a usual ring shape, a hollow column shape having an annular cross section (i.e. cylinder) of which height is relatively small or very small (see FIG. 1).
In the above process, the steps (1) to (3) may be carried out separately beforehand so as to prepare the feed mixture (for the molding) of the lubricant and the magnetic powder composite which has the predetermined particle size range and which is made of the magnetic powder (for example Fexe2x80x94Bxe2x80x94R based magnetic powder) and the resin composition.
Therefore, the present invention provides a process of producing a ring shaped resin bonded magnet or magnetic member by molding a feed mixture for molding which contains a lubricant and a magnetic powder composite made of magnetic powder (such as Fexe2x80x94Bxe2x80x94R based magnetic powder) and a heat curable resin composition which is in a solid state at a room temperature, the magnetic powder composite having a predetermined particle size range, the process comprising the steps of:
(i) filling an annular cavity of a mold with the feed mixture for molding by means of its powder fluidity;
(ii) heating an inner peripheral surface of the annular cavity so as to transfer heat to the feed mixture through the inner peripheral surface, whereby heating the feed mixture to a temperature not lower than a softening temperature and below a curing temperature of the resin composition, and compressing the filled feed mixture after or during the heating;
(iii) cooling the inner peripheral surface of the annular cavity so as to transfer heat from the heated and compressed feed mixture through the inner peripheral surface, whereby cooling the feed mixture to a temperature below the softening temperature of the resin composition so as to obtain a green compact having a ring shape;
(iv) releasing and taking out the green compact from the cavity, and
(v) heating the green compact to a temperature not lower than the curing temperature of the resin composition.
Further, the present invention provides a ring shaped resin bonded magnetic member which is produced by the above present process and which has a high density (for example, the density is more than 80% of a true density of the magnetic powder), a sufficient mechanical strength (for example, a radial crush strength is not less than 4 kgf/mm2) and good dimensional accuracy.
In the present process, during the heating of the filled feed mixture (step (5) or (ii)), heat is transferred along a direction from a center toward an outer periphery of the annular cavity. On the other hand, during the cooling of the compressed feed mixture (step (6) or (iii)), heat is transferred along a direction which is opposite to the direction during the heating.